1. Field of the Invention
This invention relates to telescopes, and particularly to an improved three-mirror telescope.
2. General Description of the Prior Art
The performance of a conventional high quality telescope when used on the earth for celestial viewing is principally limited to the earth's atmosphere rather than by the construction of the telescope. Atmospheric effects not only limit the resolution of a telescope to approximately one arc second, but also absorb large portions of the electromagnetic spectral range. While a decade or so ago little could be done about this limitation, today, with the help of earth satellites and other space vehicles, it is possible to place a telescope above the atmosphere and perform extraterrestial observations without interference from it. As a result, there has arisen a need for a telescope which can more fully take advantage of this new environment. Sought is a telescope which has a higher resolution over a wider field of view (equal to or greater than 1.degree.) and one having a greater spectral range, ideally extending from the far ultraviolet to the far infrared. It is immediately seen that to obtain the latter feature, the telescope must be an all-reflective type in order to avoid lens transmission losses, particularly at extremes of the spectral range. The first choice from existing types of reflecting telescopes would appear to be the Richey-Chretien telescope, which is an improved version of the classical Cassegrain telescope. The Ritchey-Chretien telescope is a two-mirror telescope and can, unfortunately, provide a high resolution field of only a few arc min, and it outputs a curved image field. To widen and flatten the field of this telescope, it is normally used in conjunction with refractive correctors, but refractive correctors are essentially operative only in the visual portion of the spectrum, and thus the desired wider spectral range cannot be obtained with this telescope.
As perhaps a most logical step toward improvement, several three-mirror telescopes have been proposed as possible solutions, and these have been described in the following publications:
1. Paul, M. Revue D'Optique, 14, No. 5, 1935, p.13.
2. Baker, J. E., IEEE Trans. Aer. E1. Sys., Vol. AES-5, No. 2, 1969.
3. Lagrula, Jr., Cahiers de Phys., 1942, pp. 8-43.
4. Korsch, D., Appl. Opt., Vol. 11, No. 12, 1972, p. 2986.
5. Shack, R. V. and Meinel, A.B., J.O.S.A., 56, 1966, p. 545.
6. Rumsey, N. J., Proceedings, Opt. Instru. and Techniques, 1969, Oriel Press, p. 514.
7. Buchroeder, R. A. and Leonard, A. S., Appl. Opt., Vol. 11, No. 7, 1972, p. 1649.
8. Buchroeder, R. A., Technical Report No. 68, Opt. Sc. Center, University of Arizona, 1971.
9. Korsch, D., J.O.S.A., Vol. 63, 1973, p. 667.
10. Korsch, D., Appl. Opt., Vol. 13, 1974, p. 1767.
Despite these efforts, an examination of the three-mirror systems proposed by them indicates that none of them provide practical and sufficient solutions to the problems involved, each of the systems having one or more of the following short-comings:
1. Inaccessability of the image plane.
2. A large central obscuration.
3. Practically invariable fast focal ratios.
4. A largely asymmetric configuration.
In addition to the three-mirror telescopes disclosed by the foregoing references, two additional three-mirror telescopic designs have more recently appeared, one by the Itek Corporation, described in "Requirements and Concept Design for Large Earth Survey Telescope for SEOD", Final Report, NASA CR-144796 (1975), and one by the applicant, described in Optical Engineering, Vol. 14, No. 6, (1975), p. 533. While these mirror designs offer some advantage over previous ones, they still suffer the disadvantage that, because of their geometric configurations, only less than half of the well-corrected field can be used.